1. Field of the Invention
The present invention relates to a tilt servo control device and a tilt servo control method for compensating the tilt angle of a recording surface of an optical recording medium as defined by the angle between the normal to the recording surface of the recording medium and the optical axis of a light beam irradiating the recording surface.
2. Description of the Related Background Art
In order to correctly read information recorded on an optical disk such as a DVD (Digital Versatile Disk), it is necessary to irradiate a reading light beam perpendicularly relative to a recording surface of the optical disk. If the optical disk has a warp, or errors in the mechanical system are large, it is impossible to irradiate the reading light beam perpendicular to the recording surface of the optical disk, thereby deteriorating reading accuracy for the recorded information.
In view of this problem, information reproducing apparatus for reproducing information recorded on an optical disk are normally provided with a tilt servo control device for detecting the tilt between a pickup as information reading means of the apparatus and the optical disk and compensate the tilt by adjusting the direction of the pickup in accordance with the detected tilt or by providing a signal read out by the pickup with a tilt correcting process corresponding to the detected tilt, so as to suppress the deterioration in the information reading accuracy.
Further, in order to compensate the tilt, a tilt servo control device provided with a liquid crystal panel inserted into the optical axis is known (For example, Japanese Patent Laid-Open Publication No. Hei. 11-3531). In the case of the device, the liquid crystal panel is divided into a plurality of regions, and a phase difference is provided to a light beam passing through each of the regions of the liquid crystal panel so as to maximize the level of a signal (RF signal) read out from a recording medium by a pickup.
In the case of a writable optical disk such as a DVD-R, a DVD-RW and a DVD-RAM, when no information is written in the writable optical disk, no signal can be read from the unwritten optical disk. Therefore, there is a problem in that it is not possible to properly perform the tilt servo control device for the unwritten optical disk.